Constructed Wetlands & Water Treatment
January 29, 2018
Natural wetland systems have often
been described as the “earth’s kidneys” because they filter pollutants from
water that flows through on its way to receiving lakes, streams and oceans.
Because these systems can improve water quality, engineers and scientists
construct systems that replicate the functions of natural wetlands. Constructed
wetlands are treatment systems that use natural processes involving wetland
vegetation, soils, and their associated microbial assemblages to improve water
quality.
A constructed wetland is an artificial
wetland to treat municipal or industrial wastewater, greywater or storm water
runoff. It may also be designed for land reclamation after mining, or as a
mitigation step for natural areas lost to land development.
Constructed wetlands are engineered
systems that use natural functions of vegetation, soil, and organisms to treat
wastewater. They are one example of phytoremediation. Depending on the type of
wastewater the design of the constructed wetland has to be adjusted
accordingly. It may also be necessary to use pre-treatment or post-treatment
steps.
Similarly to natural wetlands,
constructed wetlands also act as a biofilter or and can remove pollutants such
as heavy metals from the water. Some constructed wetlands may also serve as a
habitat for native and migratory wildlife, although that is not their main
purpose.
(Constructed wetlands) can be
considered treatment systems that use natural processes to stabilize,
sequester, accumulate, degrade, metabolize, and/or mineralize contaminants.
Although constructed wetland applications were limited to treating primarily
storm water and municipal wastewaters, they are now being used in new applications
and on new contaminants.
Vegetation in a wetland provides a
substrate (roots, stems, and leaves) upon which microorganisms can grow as they
break down organic materials. This community of microorganisms is known as the
periphyton. The periphyton and natural chemical processes are responsible for
approximately 90 percent of pollutant removal and waste breakdown.
A constructed wetland is an engineered
sequence of water bodies designed to filter and treat waterborne pollutants
found in sewage, industrial effluent or storm water runoff. Constructed
wetlands are used for wastewater treatment or for greywater treatment, and can
be incorporated into an ecological sanitation approach. They can be used after
a septic tank for primary treatment, in order to separate the solids from the
liquid effluent.
The plants remove about seven to ten
percent of pollutants, and act as a carbon source for the microbes when they
decay. Different species of aquatic plants have different rates of heavy metal
uptake, a consideration for plant selection in a constructed wetland used for
water treatment. Constructed wetlands are of two basic types: subsurface flow
and surface flow wetlands.
The main three constructed wetlands
types are:
• Subsurface
flow constructed wetland - this wetland can be either with vertical flow (the
effluent moves vertically, from the planted layer down through the substrate
and out) or with horizontal flow (the effluent moves horizontally, parallel to
the surface)
• Surface
flow constructed wetland
• Floating
treatment wetland
The planted vegetation plays an
important role in contaminant removal. The filter bed, consisting usually of
sand and gravel, has an equally important role to play.
Subsurface
flow constructed wetlands
Subsurface flow wetlands can be
further classified as horizontal flow and vertical flow constructed wetlands.
In the vertical flow constructed wetland, the effluent moves vertically from
the planted layer down through the substrate and out (requiring air pumps to
aerate the bed). In the horizontal flow CW the effluent moves horizontally via
gravity, parallel to the surface, with no surface water thus avoiding mosquito
breeding. Vertical flow CWs are considered to be more efficient with less area
required compared to horizontal flow CWs. However, they need to be
interval-loaded and their design requires more know-how while horizontal flow
CWs can receive wastewater continuously and are easier to build.
Subsurface flow wetlands can treat a
variety of different wastewaters, such as household wastewater, agricultural,
paper mill wastewater, mining runoff, tannery or meat processing wastes, storm
water.
The quality of the effluent is
determined by the design and should be customized for the intended reuse
application (like irrigation or toilet flushing) or the disposal method.
Surface
Flow Constructed wetlands
Surface Flow Constructed wetlands
(SFCW) Also called Free Water Surface CWs (FWS) Biological activity takes
mainly place in the superior layer of the soil, in the stems of the plants and
in the water. Waterproofing is not always used. SFCW are birthing grounds to
mosquitoes and require greater protection from public access than Subsurface
Flow CWs.
In a free-surface constructed wetland
(also known as surface flow CW or free water surface CW), water flows above
ground and plants are rooted in the sediment layer at the base of the basin or
floating in the water. As the water slowly flows through the wetland,
simultaneous physical, chemical and biological processes filter solids, degrade
organics and remove nutrients from the wastewater. The channel or basin is
lined with an impermeable barrier (clay or geo-textile) covered with rocks,
gravel and soil and planted with native vegetation (e.g., cattails, reeds
and/or rushes). The wetland is flooded with wastewater to a depth of 10 to 45
cm above ground level. The wetland is compartmentalized into at least two
independent flow paths. The number of compartments in series depends on the
treatment target. The efficiency of the free-water surface constructed wetland
also depends on how well the water is distributed at the inlet. Wastewater can
be fed into the wetland, using weirs or by drilling holes in a distribution
pipe, to allow it to enter at evenly spaced intervals.
Floating
System Wetlands
Floating treatment wetlands (FTWs) are
manmade ecosystems that mimic natural wetlands. FTWs are created using floating
rafts that support plants grown hydroponically. The rafts float on a wet pond
water surface and can be used to improve water quality by filtering, consuming,
or breaking down pollutants (e.g., nutrients, sediment, and metals) from the
water.
If it can be demonstrated that FTWs
effectively remove waterborne pollutants, FTWs could be placed on most existing
lakes and ponds. Many of these ponds located in urban settings are used as storm
water catchments.
Nitrogen removal by nitrification/denitrification is the process
mediated by microorganisms. The physical process of volatilization also is
important in nitrogen removal. Plants take up the dissolved nutrients and other
pollutants from the water, using them to produce additional plant biomass. The
nutrients and pollutants then move through the plant body to underground
storage organs when the plants senesce, being deposited in the bottom sediments
through litter and peat accretion when the plants die. Wetland microorganisms,
including bacteria and fungi, remove soluble organic matter, coagulate
colloidal material, stabilize organic matter, and convert organic matter into
various gases and new cell tissue. Many of the microorganisms are the same as
those occurring in conventional wastewater treatment systems. The effectiveness
of all processes (biological, chemical, physical) varies with the water
residence time (i.e., the length of time the water stays in the wetland).
Longer retention times accelerate the remove of more contaminants, although
too-long retention times can have detrimental effects.
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